It is generally common knowledge that the United States car industry began to substantially reduce the size of their models in the decade of the 70's. This down sizing of cars resulted in design features which ultimately affected the use of the then conventional Type `P` tire chain. The principal design feature affecting the use of conventionsl Type P tire chain was restricted fender clearances. As a result of the restricted clearance, some type P tire chains would impact portions of the fender areas on some cars. Neither the cars nor the tire chains benefited from the impacts. Ultimately, in 1979, the National Association of Chain Manufacturers defined a specification (NACM-5179(TC) dated May 1, 1979) defining a Type `PL` tire chain. The Type `PL` tire chain was specified for applications requiring SAE Class S traction devices. The Class S traction device has the most constrained dynamic profile now defined by the SAE.
To achieve this profile, which reduced the maximum extension of the tire chain from the tire center slightly more than twenty-six percent, a number of changes to the NACM standards were made. Basically, for a given tire size, the side chain length was slightly reduced, the cross chain lengths were slightly increased and the wire stock size decreased. The reduction in wire stock size of the side links was about eight percent and the cross member links were reduced about fourteen percent. The steel and heat treatment of the `PL` links were unchanged from previous standards. Typically the industry continued to use a low carbon steel, often AISI 1008, as the link material. The links were case hardened to a depth of about 1/4 to about 1/18 of the wire diameter and to a surface hardness of at least Rockwell C53.
Unfortunately, when the new Type `PL` chains were put into service, they proved to be less durable than the traditional Type `P` chain. When used on a traditional rear wheel drive car durability was reduced about 25%. Contemporaneously with car down sizing, domestic manufacturers increasingly turned to front wheel drive cars. The durability of Type `PL` chains on a front wheel drive vehicle was reduced about an additional 25%.
The reduction of durability in the Type `PL` chain to about 50% of that experienced with the conventional Type `P` chain, in many applications, caused considerable consternation.
Extra durable chain was manufactured primarily for truck use, by at least Campbell Chain Division of Cooper Industries, the assignee of the present invention. Campbell's extra durable truck tire chain was manufactured from alloy steel, AISI 4615 or 8620, which obtained a distinctive property of durability chiefly from heat treating and from the addition of nickle or chromium rather than carbon. Normally, alloy steel was used for high strength chain in overhead lifting application and had prepared link end welds. This type alloy chain was not generally case hardened. The extra durable tire chain was case hardened, liquid quenched, and the links were closed conventionally by prepared end welding adjacent link ends. The relatively high cost of obtaining durability in a Type `PL` tire chain by employing alloy steel of the extra durable type would result in a prohibitive price to the consumer for a car tire chain.
Butt welding links to close them is conventional in tire chain. Because butt welding can result in internal voids, or inclusions, or both, prepared end welds have often been employed in chain suitable for overhead lifting applications. Likewise tempering of either alloy or medium carbon steel has often been employed in high strength chain. Most often tempering has been employed in overhead lifting chain.
The fabrication and material of chain is dependent on the intended application envisioned for the chain. Material selection among, for example, low, medium, or high carbon steel or alloy steel is not always straight forward to obtain a given property needed in a particular application. It is believed that a number of the members of the tire chain industry independently attempted to determine an optimum ratio between case depth and stock diameter to maximize durability. The results of their investigations were inconclusive. A number believed that case depth should be deep, on the order of 1/4 of wire diameter, but other believed in shallow depths, of about 1/8. When the selections in the design process include commercial factors as well as choices among tempering techniques or case hardening techniques predicting the success of a design becomes so complex that empirical techniques must often be resorted to.
Although the industry was aware of the durability problem associated with conventional Type `PL` tire chains a recognized solution was not found.